Dennis Quinlan (the 'Velobanjogent' - a Frankenlogo if ever there was one!) sent this clipping from the April 1956 edition of the New South Wales Motorcyclist, presumably a 'modern' motorcycle publication which occasionally featured Vintage motorcycles...although the Rover in the photograph is a Veteran in the VMCC system of 'eras'. The 'Veteran' being anything with two wheels and a motor built before 1919, the 'Vintage' era from 1919 - 1930 (the last age of Nickel plating), the 'Post-Vintage' 1931-47, the 'Classic' 1948 onwards... and things start to get fuzzy in the 'modern' era. When the Vintage MCC was founded in 1948 in England, anyone showing up on a 40s machine was frowned upon! Of course now they're considered as important historically as the Veterans.
The VMCC originally considered fixing their eligibility dates as pre-1947, end of story, but wiser heads prevailed and they, like the AMCA here in the States, have a rolling '35 Year' rule for inclusion. Which means of course, that your Kawasaki Z1 is now eligible for the VMCC and AMCA events; strange bedfellows with a 1915 Harley or 1908 Rover!
As for the origins of the term 'Vintagent'; having been asked many times, my research has proved inconclusive - I remember reading columns by Bob Currie when he wrote for Classic Bike in their toddler years (the early 1980s), and understand Titch Allen used the term freely. It's likely when the VMCC was formed, some clever English wag concocted the name, and it stuck, as it perfectly suits 'us'.